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The effect of ventilation hypercapnia on pulmonary circulation in man was investigated through separate studies. In the first study on 44 patients with little or no airway obstruction and 20 normal men, 5 % CO₂ breathing produced (a) significant rise in pulmonary artery pressure (PAP), (b) no significant change in cardiac output, (c) rise in pulmonary vascular resistance, (d) rise in brachial artery pressure (BAP) and (e) no change in wedge pressure (WP). The rise in PAP was more pronounced after 2 min of 10% CO₂ breathing in 12 bronchitics. The second study was carried out in 39 bronchitics and 22 normals while breathing 10% CO₂ for 1 min and showed that pulmonary vascular response was independent of systemic vascular response, in that BAP rose later and came back earlier to original level during CO₂ breathing. In the third study on 26 severe bronchitics and 15 normals the observed rise in PAP during 10% CO₂ breathing was independent of H-ion concentration in the blood since PAP continued to rise even when pH was maintained at air breathing level by intravenous injection of 130 mEq of sodium bicarbonate in 250 cm3 of 5% glucose solution. This study also confirmed the findings in the first study that there was minimal rise in cardiac output, no rise in WP, while PAP and pulmonary vascular resistance rose significantly during ventilation hypercapnia. The responses were pronounced compared with those observed in the first study with 5 % CO₂. It is postulated that the responses might be due to direct action of CO₂ on muscular pulmonary arteries.

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Lung scanning was performed in 12 subjects who had pulmonary oedema of high altitude (POHA) before, and in 8 men who had been at high altitude without developing pulmonary oedema. Scanning done before and after 5 min 10% O₂ breathing showed (a) increased perfusion in POHA subjects after hypoxia, (b) more in the lower than in upper zones, and (c) striking increase in perfusion in localised areas. In controls, however, there was significant reduction in perfusion after hypoxia in most areas. The reduction was less marked in the lower zones than in the upper zones. POHA subjects were hyperreactors to hypoxia, compared to controls. It is concluded that scanning before and after hypoxic breathing might help in differentiating hyperreactors.